1. Field of the Invention
This invention relates to a biological information measurement apparatus and, more particularly, to a biological information measurement apparatus for measuring biological information safely and easily by contacting a probe with the surface of a living body or inserting the probe directly into the living body.
2. Description of the Related Art
In general, with a biological information measurement apparatus of this kind, particularly a measurement apparatus for medical purposes, safety measures are of paramount importance, in comparison with other types of industrial measurement apparatus. Since a commercial (AC) power supply is used in a biological information measurement apparatus, there is the possibility that a patient or operator will receive an electric shock if electrically connected to the commercial (AC) power supply. The greatest danger involving this electric shock is that of ventricular fibrillation caused by an electric current flowing into the body. Death will result if such ventricular fibrillation is left to continue for two to three minutes. In particular, in measurement of cardiac output of the type in which a catheter is inserted directly into the heart, an accident can cause all of the electric current to flow into the heart through the catheter. For this reason, safety must be given the greatest consideration. In this case, it has been reported that a safe value of current that can flow directly into the heart is on the order of 10 .mu.A-20 .mu.A. It has also been reported that the value is less than 500 .mu.A even between skins having a comparatively high resistance. In this respect, conventional safety measures involve using, say, a power supply transformer provided with double insulation to sufficiently cut off the power supply circuit of the apparatus main body from the commercial (AC) power supply circuit, and grounding the apparatus main body. Conventional safety measures also include grounding one side of a measurement electrode circuit.
However, these one-sided safety measures alone provide no assurance that minute leakage on the order of tens of microamps can be prevented at all times. There is also a type of measurement in which current must be positively passed into a detecting circuit such as a probe. Relying upon a transformer at one location for the purpose of isolation cannot be considered a sufficient safety measure in view of problems involving the layout of the apparatus contacting the power supply section, the aging of the apparatus and the environment in which the apparatus is used. In addition, a living body is not always at ground potential, and there are cases where forcibly placing a living body at ground potential can be more dangerous instead.
In recent years, various types of electronic sensors (temperature sensors, light sensors, pressure sensors and ion sensors, etc.) have been developed, and these sensors are widely applied in the aforementioned biological measurement apparatus, particularly in a variety of electronic measurement apparatus for medical purposes. Most of these sensors utilize a change in the electrical characteristics (quantity of electricity) of a sensor element, but in general it is difficult to make the electrical characteristics perfectly uniform for each and every sensor element. Consequently, the practice in the prior art is to select and use those sensor elements whose electrical characteristics are most uniform. An alternative practice is to correct for variances in electrical characteristics of sensor elements in each and every measurement apparatus by using a corrective characteristic element.
In general, since a probe is easily consumed, it is desired that probes be made readily replaceable. However, when it is attempted to effect a correction for each and every probe in advance, there is a limitation upon this type of correction. Therefore, when a measurement requiring high precision is carried out, conformity between the probe and the main body of the measurement apparatus becomes a major problem.